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Editorial Team
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Asked: 2026-05-24T03:46:04+00:00

When compiler optimizes code, what is the scope of the optimizations? Can an optimization

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When compiler optimizes code, what is the scope of the optimizations? Can an optimization
a) span more than a set of nested braces?
b) span more than a function?
c) span more than a file?

We are chasing an obscure bug that seems to derive from optimization. Code crashes in release mode but not debug mode. And of course we are aware this could be heap corruption or other memory problem bur have been chasing this for a while now. One avenue we are considering is to selectively compile our files in debug mode until the problem goes away. In other words:

a) Start with all file compiled in release mode
b) Compile 1/2 the files in debug mode
if crash still seen, take half the release compiled files and compile in debug mode
if crash not seen, talk half the files compiled in debug mode and compile in release mode
repeat until we narrow in on suspect files
this is a binary search to narrow in on problem files

We are aware that if this is a memory issue, simple doing this mixed compilation may make the bug go away, but we are curous if we can narrow in on the problem files.

The outstanding question though is what is the scope of optimizations – can they span more than one file?

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1 Answer

  1. Editorial Team

    An optimization can do literally anything as long as it doesn’t change the semantics of the behaviour defined by the language. That means the answers to your first questions (a), (b), and (c) are all yes. In practice, most compilers aren’t that ambitious, but there are certainly some examples. Clang and LLVM have flags for link time optimization that allow optimizations to span pretty much the entire program. MSVC has a similar /GL flag that allows whole-program optimization.

    Often the causes of these sorts of failures are uninitialized variables. Static analysis tools can be very helpful in finding problems like the one you’re describing. I’m not sure your binary searching via optimizations is going to help you track down too much, though it is possible. Your bug could be the result of pretty complicated interactions between modules.

    Good luck!

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